Materials that can be electrically actuated are based on a variety of physical responses of the polymer to an applied voltage. These materials include dielectric elastomers, ferroelectric polymers, and conducting polymers. Materials with fast response times and high strains at relatively low applied voltages would be useful. Liquid crystal elastomers (LCEs), which incorporate the anisotropy of liquid crystal molecules, produce reversible macroscopic shape changes under thermal or optical stimulation. Nanometer-scale voltage-induced actuation has been demonstrated in a smectic elastomer.
New high performance actuator materials capable of generating large mechanical actuation induced by external stimuli such as electric field and temperature are needed for a wide range of applications from MEMS to robotics. Materials with high strain levels coupled with high energy density are needed for these applications. Supramolecular ordered assemblies such as liquid crystals provide an excellent framework for incorporating anisotropy as well as functionalities in materials that respond to external stimuli. Nematic liquid crystal elastomers have been extensively studied in the past. These materials undergo macroscopic form change as a result of the order-disorder transition of the mesogens.) This effect could form the basis of an artificial muscle (see de Gennes, et al, Macromol. Symp. 113, 39 (1997) and Thomsen et al., Macromol. 34, 5868 (2001). In the case of these elastomers, a temperature driven phase transition from nematic phase to isotropic phase causes a uniaxial contraction of the elastomer. On heating the film, a remarkable contraction of up to a factor of 4 occurs in the isotropic state. Despite this, these materials suffer from slow thermoelastic response due to the inherently low thermal and electrical conductivity of the elastomer.
The theoretical mechanism for shape changes in liquid-crystalline elastomers is based on a coupling between the orientational order of the mesogenic groups and distortions of the crosslinked polymer network. Because of this coupling, any changes in the magnitude or direction of the orientational order affect the macroscopic shape of the elastomer and, conversely, any distortions in the shape of the elastomer affect the orientational order of the mesogenic groups (see Warner, et al, Liquid Crystal Elastomers, Oxford University Press, 2003). In particular, the polymer network tends to extend along the direction of orientational order, and to contract in the two transverse directions.
An alternative approach is to use liquid crystalline elastomers that exhibit chiral smectic A (electroclinic) phase. Garoff et al, R.B. Physical Review A 18, 2739, 1978, first demonstrated that when an electric field is applied to a smectic A liquid crystal composed of chiral molecules along the layer plane, the transverse dipole of the molecules couple to the electric field and tilt the molecules in a plane perpendicular to the electric field direction. This field-induced tilting of molecules is known as the electroclinic effect. It is also referred to as the “soft mode” in analogy with the softening of a vibration mode near the paraelectric-ferroelecric transition in solid ferroelectrics like barium titanate. The electroclinic effect and the induced tilt angle θ of the molecules increases continuously with the field. The tilt leads to a layer contraction Δl proportional to (1-cos θ) which cumulatively should result in a macroscopic contraction of the sample in a direction perpendicular to the smectic layers and a concomitant extension parallel to the smectic layers. The extension is expected to scale with a sin θ dependence on the tilt angle. Lehmann et al. [Lehmann, et al. Nature, 410, 447, 2001] demonstrated that the thickness of a freely suspended ultra-thin (less than 100 nm thick) electroclinic elastomer film indeed decreases due to the layer contraction.
The advantage of the elecroclinic approach is that electric fields can be used to induce shape variation based on the electroclinic effect. Moreover, these materials require low switching voltages and exhibit fast switching speed at 2-3 orders of magnitude faster compared to nematic elastomers. An application of an electric field parallel to the smectic layers induces a tilt θ of the mesogens that varies continuously with field. This tilt leads to a contraction of the sample perpendicular to the smectic layers and an elongation parallel to the smectic layers.
The layer contraction and film extension may be understood from geometrical considerations, because the change in layer spacing, Δl, is proportional to the cosine of the tilt angle θ while the extension is proportional to the sine of the tilt angle (FIG. 1). The degree of contraction and extension of the elastomers is proportional to the magnitude of the electroclinic tilt. Note that this distortion depends on the sign of the applied electric field; reversing the sign of the field inverts the shape of the distortion.
While there have been quite a few investigations on the mechanical properties of nematic elastomers, the mechanical properties of chiral smectic A elastomers has been studied much less. Recently, ferroelectric liquid crystal elastomers showing actuation under an applied electric field have been demonstrated. Lehmann et al. [Nature 410, 447, 2001] and Kohler et al. [Appl. Phys. A-Mater. Sci. & Proc. 80, 381, 2005] showed an electrostriction of an ultra-thin film (less than 100 nanometers) that exhibits up to a 4% strain. In both of these studies, the ultra thin membranes were not practical for use as shape changing membranes for several reasons. First, the thickness of the films makes them extremely difficult to prepare and handle in any other setup than the specific experimental method they describe, thus severely limiting their use outside of the specific method they employ. Second, the orientation of the films, which were prepared with the liquid crystal molecules in homeotropic alignment in reference to the electrodes, will not function as an actuator for practical applications since the contractile strain occurs across the film thickness. In addition, the preparation the elastomer films rendered the film thickness non-uniform, thus creating non-uniform strains throughout the film when subjected to applied fields. Finally, preparation of the films imposed strict limitations on the manner in which the solid electrodes were positioned and attached to the film. Since the electrodes must be positioned at the edge of the homeotropically aligned film, it is not amenable to patterning or scaling. Introduction of flexible electrodes is also not possible with the experimental setup they describe. Due to the film preparation in these studies, the limitations in the position of the electrodes with respect to the film introduced large gradients, inhomogeneities, and fluctuations in the electric field applied to the films. It is apparent from the experimental design that non-obvious changes in the operation and preparation of the films must be introduced in order to reduce the electric actuation of the films to practice.
Thus there is a need in the art for new high performance actuator materials capable of generating large mechanical actuation induced by external stimuli. These materials need to be precisely controlled. These and other needs are met by the present invention.